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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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202                                ALASKAN GLACIER STUDIES
able influence, the ice and debris supply which they contribute being so small a proportion of the whole that they are completely dominated. Occasionally a tributary is so nearly equal to the main stream that it maintains individuality, both in its ice currents and in its morainic areas, clear to the glacier terminus; and now and then, where two large glaciers unite, one is so much the stronger that, as in Hubbard, Nunatak, and Fourth Glaciers, both the ice current and the moraine of one are pushed far over to one side by the greater force of the other. On the other extreme we find many cases of glaciers, especially those cascading from small hanging valleys, which barely coalesce with the main glacier, and sometimes just fall short of doing so. In such cases local enlargement of the lateral moraine of the main glacier occurs at and just below the point of incoming of the tributary, but otherwise no noticeable influence is caused by it.
Some glaciers terminate in hanging valleys, at varying distances back from the lips. Such glaciers are often moraine-covered at their ends, and, if they halt long enough, they build terminal moraines. From the terminus one or more streams issue, heavily charged with de'bris, which in some cases, where the valley grade is steep enough, is borne on beyond the lip of the hanging valley, but in other cases is partly deposited on the floor of the hanging valley, forming a more or less perfect outwash gravel plain. In all cases a large porportion of the debris is borne on to the lip of the valley, thence down the steepened slope to the flatter slope at its base, or to the sea, where deposit is extensive. Here are built steep alluvial fans on valley sides, and large, steep deltas where the steepened slope descends to the fiord. The great volume of water, with its sediment load and its steep grade has invariably cut a gorge below the hanging valley lip which it is rapidly deepening; but the time since these streams have been flowing is so brief that the gorges are rarely deep or broad. They increase in depth and breadth toward the outer portions of the fiord, as one would expect in view of the fact that the steepened slopes there have longer been exposed to the erosive action of the stream; but this increase is less striking than might be expected, since the glaciers in these hanging valleys are smaller than those in the inner portions of the fiord and the water supplied by their melting is therefore less in quantity.
A few hanging valley glaciers extend to the very lip of the valley, and some extend even beyond it, terminating as cascading glaciers. In these cases there is a smaller alluvial fan or delta at the base of the steepened slope, and a less developed gorge has been cut in the steepened slope. Indeed, the cascading glaciers may have no gorge whatsoever, their streams flowing along several courses on the surface of the steepened slope. These glaciers usually have a more or less well-defined moraine at the nHff base in which is incorporated snow and ice blocks that have fallen from the steeply perched ice terminus. Being only recently disconnected from the main glacier, and having usually only a brief duration in steeply-perched positions, the cascading glaciers do not commonly form extensive deposits or leave striking records of their former presence; and such records as they do leave are soon buried beneath the growing alluvial fans.
Fairly good sized glaciers that terminate in valleys near sea level, where ablation is rapid, have made much more extensive deposits than in any of the preceding cases. If the glacier fronts were stationary, terminal moraines would doubtless develop; but, since the glacier history of Yakutat Bay has been one of general recession, such deposits are not common, though they have been found in a few places, such as Calahonda valley. The receding glacier contributes most of its debris to streams, leaving only t.Tn'n deposits